This disclosure is directed to methods and apparatus for identifying a region of interest in a medical image.
The definition of regions or volumes of interest (ROI/VOI) is a typical precursor to quantitative analysis of medical images, such as nuclear medicine emission images (for example, PET or SPECT). Such regions may be defined around areas of high intensity which correspond to high tracer uptake (hotspots). For example, in FDG-PET images for oncology, such areas may be indicative of the presence of a tumor. Oncology physicians frequently annotate lesions in PET scans for the purpose of making a diagnosis, or for use in radiotherapy. The mean or maximum tracer uptake can aid a reader in determining the likelihood of cancer. In longitudinal studies, considering the change in intensity or uptake on corresponding VOIs from images at different temporal stages may be used to determine whether a tumor has regressed.
The ROI/VOI delineation step is generally a user interactive process. In PET, it is common to define such regions using a manually adjusted threshold either defined on an absolute scale or with reference to a local maximum in intensity, or some other reference region.
In most applications, this is typically achieved by selecting a region and a threshold that delineates the lesion. One issue in such threshold based segmentations is the determination of the threshold. It should be determined such that the object of interest is included in the VOI but such that the background is not. In some cases this determination is made difficult by the presence of other high uptake structures or features of the image adjacent to the region of interest. For example a lung tumor may be close to the heart left ventricle, a site of typical high uptake in FDG-PET. Alternatively, there may be several tumors in close proximity to one another and the user may wish to delineate each separately.
The difficulty is more pronounced in 3D than in 2D since the user must check each slice over which the VOI is defined, since connectivity between voxels included in the object of interest may be present across voxels not in the current displayed slices. This can be slow and laborious.
More advanced delineation methods exist. One of them is the Incremental Connected Components (ICC) method described in the applicant's co-pending UK patent application no. 0914637.4 corresponding to US published application 2010/0088644, incorporated herein by reference. This method defines a set of regions associated with local maxima in an image. Its output is a description of the topology of these local maxima and allows rapid selection of hotspots (hereafter called Regions of Interest (ROIs)) near a click point given a threshold. The ICC algorithm aims to guarantee the selection of an ROI that contains only one connected component, a fundamental characteristic of the algorithm.
However, even such methods can produce erroneous or unsuitable ROIs, usually depending on the threshold selected by the algorithm, or by the user.